Time of flight ranging systems find use in level measurements applications, and are commonly referred to as level measurement systems. Level measurement systems determine the distance to a reflective surface (i.e. reflector) by measuring how long after transmission energy, an echo is received. Such systems may utilize ultrasonic pulses, pulse radar signals, or other microwave energy signals.
Pulse radar and microwave-based level measurement systems are typically preferred in applications where the atmosphere in the container or vessel is subject to large temperature changes, high humidity, dust and other types of conditions which can affect propagation. To provide a sufficient receive response, a high gain antenna is typically used. High gain usually translates into a large antenna size with respect to the wavelength.
Two types of antenna designs are typically found in microwave-based level measurement systems: rod antennas and horn antennas. Rod antennas have a narrow and elongated configuration and are suitable for containers having small opening/flange sizes and sufficient height for accommodating larger rod antennas. Horn antennas, on the other hand, are wider and shorter than rod antennas. Horn antennas are typically used in installations with space limitations, for example, vessels or containers which are shallow.
The level measurement instrument or device comprises a housing and a waveguide (i.e. the antenna). The level measurement instrument is mounted on top of a container or vessel and the antenna extends into the vessel. The level measurement instrument is typically bolted to a flange around the opening of the container. The housing holds the electronic circuitry. The antenna extends into the interior of the vessel and is connected to a coupler which is affixed to the housing. The antenna is electrically coupled to the electronic circuit through a waveguide, for example, a coaxial cable. The waveguide has one port connected to the antenna coupler and another port connected to a bidirectional or input/output port for the electronic circuit. The antenna converts guided waves into free radiated waves, and is reciprocal, i.e. also converts the free radiated waves into guided waves. The antenna is excited by electromagnetic (i.e. radio frequency) pulses or energy received through the waveguide from the circuit and transmits electromagnetic pulses or energy into the vessel. The antenna couples the pulses that are reflected by the surface of the material contained in the vessel and these pulses are converted into guided electromagnetic signals or energy pulses which are guided by the waveguide to the circuit.
In many applications, the material contained in the vessel and being measured is held at high temperatures and/or high pressures. Furthermore, the material itself may comprise highly aggressive (i.e. highly corrosive) chemicals or substances. It will be appreciated that such substances or conditions present a harsh operating environment for the level measurement device and, in particular, the process interface between the antenna and the material.
Accordingly, there remains a need for improvements in a horn antenna configuration and/or emitter structure for radar-based level measurement systems.